This invention relates to horizon sensors, and more particularly to a method and apparatus for compensating for radiance variations of the earth's horizon.
Horizon sensors are devices which are mounted in satellites, space probes and the like which sense the earth's horizon and produce signals which indicate orientation of the satellite with respect to the earth. The signals are derived by scanning an infrared detector over the earth's horizon which is extremely warm compared to cold outer space generating a signal on such crossings used along with reference signals as the scan passes predetermined points in the orbit of the orbiting space craft. The intervals between the horizon crossings and the reference pulses are used to produce an output signal providing attitude information of the orbiting body with respect to the earth. The infrared detector is sensitive to radiation from the carbon dioxide band in the 14-16 micrometer range because of the uniformity of the horizon profile over all atmospheric conditions. However, radiation from the earth is not uniform and varies depending on the month of the year and location of the scanning pattern which if not corrected or considered will produce errors in locating the true horizon. Accordingly, compensation must be provided for radiation variations in order to improve the accuracy of the horizon sensor. This is true whether the scanning is being done by the satellite itself which rotates in orbit and scans an infrared detector mounted thereon across the horizon as it rotates in orbit around the earth or a rotating optical element is mounted onto the satellite and scans the detector across the horizon. For example, in U.S. Pat. No. 3,020,407 a rotating optical prism scans the infrared detector in a conical scan pattern across the horizon to derive the electrical output signals marking the line of thermal discontinuity between cold outer space and the warm earth. In addition, the earth's horizon must be distinguished from other celestial objects such as the sun or the moon which also generate signals if they appear in the scanning pattern of the infrared detector. Accordingly, many such horizon sensors use a separate detector or means for eliminating the effects of the sun so that false horizon indications will not occur.